From the Camel (3rd ed.) pp. 87-88, you'll see that && has a higher precendence than =. So, your statement actually looks like:

$idx{$val} ||= (1 && push(@num, $val));
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Then, and only then, does the optimizer deal with the '1 &&' part. This is in contrast to using 'and' instead of '&&', which binds looser than = (and its sibling ||=). Try it out!